1. Field of the Invention
The present invention relates to a fuel injection pump for use mainly in a diesel engine, and more specifically, to an injection timing control device for a distributor type fuel injection pump, including a fuel-pressure control valve which is operated under the command of an electronic control unit (ECU) for controlling the fuel injection timing of the fuel injection pump.
2. Description of the Prior Art
As disclosed such as in Japanese First (unexamined) Utility Model Publication No. 63-110640, in the distributor type fuel injection pump for the diesel engine, a timer piston is provided for changing the displacing timing of a cam. The displacing timing of the cam controls the displacing timing of a plunger which is provided for feeding fuel under high pressure. A fuel-pressure control valve is provided between pressure chambers located at opposite sides of the timer piston for adjusting a pressure differential applied across the timer piston so as to control a position of the timer piston. The position of the timer piston defines the displacing timing of the cam so that the fuel injection timing is controlled via the movement of the plunger.
In, for example, Japanese First (unexamined) Utility Model Publication No. 56-173736, a fuel-pressure control valve is in the form of a solenoid valve. Accordingly, by controlling energization and deenergization of a coil of the solenoid valve for changing a pressure differential across a timer piston so as to adjust a position of the timer piston, the fuel injection timing of the fuel injection pump can be controlled electronically.
FIG. 14 shows an example of the conventional fuel-pressure control valve as generally designated at numeral 80. Communication between passages 41 and 42 is determined by controlling energization of a coil 54. Specifically, when the coil 54 is energized, an armature 52 integrated with a valve needle 98 is attracted toward a stator 53 against a biasing force of a spring 57 to move rightward in the figure until an intermediate surface 99 of the valve needle 98 abuts a shim 100. Thus, the fuel-pressure control valve 80 is opened to establish communication between the passages 41 and 42. A valve lift l.sub.7 at this time is l.sub.7 =0.7 mm. An air gap l.sub.8 as an interval between the armature 52 and the stator 53 has a relationship with the valve lift l.sub.7 so that l.sub.8 =l.sub.8 +0.05 mm. Accordingly, l.sub.8 =0.75 mm when the valve 80 is closed, while l.sub.8 =0.05 mm when the valve 80 is opened. When the coil 54 is deenergized, the armature 52 integrated with the valve needle 98 moves leftward in the figure due to the biasing force of the spring 57 so that the valve 80 is closed.
In recent years, following the strengthening of exhaust gas regulation, the synchronous control relative to engine rotation has been required also for the oil-pressure control valve in the injection timing control device for the fuel injection pump as described in Japanese First (unexamined) Patent Publication No. 62-101865. This inevitably requires the high-speed response of the valve needle 98. However, the conventional fuel-pressure control valve 80 as shown in FIG. 14 requires a long time from energization of the coil 54 to the valve opening, that is, the valve opening response is significantly poor. Thus, the fuel-pressure control valve with an improved valve opening response has been demanded.
For improving the valve opening response of the valve 80, that is, the valve needle 98, it is essential to shorten a time period from energization of the coil 54 to generation of the sufficient attraction force. For achieving this, it is effective to reduce the number of coil turns N as shown in FIG. 15. However, when the number of coil turns N is reduced, the larger quantity of current is required for achieving the same attraction force so that a drive circuit for the valve 80 is increased in cost.
On the other hand, as shown in FIG. 16, when the air gap l.sub.8 is reduced, the larger attraction force can be achieved with the smaller current. In this case, however, since the valve lift l.sub.7 should be inevitably reduced, a sufficient magnitude of passage area can not be achieved between the passages 41 and 42 via a groove 98a of the valve needle 98.
In the meantime, due to the recent cost-reduction requirement, it has been proposed to decrease the cost of the fuel-pressure control valve, using a portion of parts of a fuel injection valve (solenoid valve) for the gasoline engine since the gasoline engines have been produced in large quantities. Examples are shown in Japanese First (unexamined) Patent Publications Nos. 60-132038 and 2-211374. FIG. 17 shows the fuel-pressure control valve described in the former publication, as generally designated at numeral 101.
In FIG. 17, during a high-pressure fuel feeding stroke of a plunger (not shown), a timer piston 21 is urged upward in the figure due to a reaction force from a face cam (not shown) applied via a slide pin 19. Thus, a timer high-pressure chamber 22 increases in pressure in proportion to fuel injection pressure during the high-pressure fuel feeding by the plunger. The timer high-pressure chamber 22 communicates with a housing chamber 102 via a passage 103 and further communicates with a passage 107 arranged surrounding a valve needle 106 via a passage 105 formed in a valve body 104. Since the passage 107 further communicates with a spring chamber 108, the spring chamber 108 is at a pressure equal to that in the timer high-pressure chamber 22. On the other hand, since a timer low-pressure chamber 24 is at a pressure which is constantly lower than that in the timer high-pressure chamber 22, a chamber 110 communicating with the timer low-pressure chamber 24 via a passage 109 is also under the low pressure. With this arrangement, when the pressure in the timer high-pressure chamber 22 is increased, the pressure in the spring chamber 108 is also increased so that the valve needle 106 is urged leftward in the figure with an increased force, which is opposite to a valve opening direction of the fuel-pressure control valve 101. Accordingly, the valve opening response of the valve 101 changes depending on the pressure in the timer high-pressure chamber 22. Thus, the fuel injection timing can not be controlled accurately.
Further, since the housing chamber 102 also increases in pressure to a high value every time the plunger achieves the high-pressure fuel feeding, the fuel-pressure control valve 101 repeatedly receives loads rightward in the figure so that bolts 111 by means of which the valve 101 is fixed to a housing 17 are subjected to fatigue failure.